1. Field of the Invention
When producing those hydrofluorcarbons which do not contain chlorine and are hence free from destruction of the ozone layer (hereinafter referred to as "HFC"), among others, difluoromethane (hereinafter referred to as "HFC-32"), 1,1,1,2-tetrafluoroethane (hereinafter referred to as "HFC-134a") and pentafluoroethane (hereinafter referred to as "HFC-125"), the present invention relates to a fluorination catalyst improved for producing the HFCs with high productivity, a production method of the fluorination catalyst, and a method of efficiently producing the HFCs by bringing hydrogen fluoride and a halogenated hydrocarbon into mutual contact in a gaseous phase by using the catalyst.
2. Description of the Related Art
A typical example of the industrial production methods of the HFCs is the method which replaces halogens other than F by F by bringing a hydrogen-containing halogenated hydrocarbon into contact with HF (in many cases, addition of HF using an unsaturated halogenated hydrocarbon as the starting material and the exchange reaction of the halogens other than F with F are simultaneously conducted). However, the reaction does not smoothly proceed in most cases, and the production quantity of the HFCs greatly depends on the catalyst used.
An example of the most difficult reaction is the synthesizing reaction of HFC-134a by fluorination of 1-chloro-2,2,2-trifluoroethane (hereinafter referred to as "HCFC-133a"), and this reaction is an endothermic reaction which is not advantageous thermodynamically. Therefore, the reaction is generally carried out at a relatively high temperature by allowing HF in an amount exceeding a stoichiometric amount to be co-present with HCFC-133a. According to Japanese Unexamined Patent Publication (Kokai) No. 55-27138, for example, HFC-134a is obtained at a yield of 32% by conducting the reaction at a temperature of 400.degree. C. using a compound obtained by treating CrF.sub.3.3H.sub.2 O with air, as the catalyst. The reaction at such a high reaction temperature promotes coking of the catalyst and reduces catalyst life. To prevent coking, an attempt was made to allow oxygen to be co-present inside the reaction gas (Japanese Unexamined Patent Publication (Kokai) No. 55-27139), but this method is not desirable because chlorinated by-products increase. To restrict the formation of the chlorinated by-products, Japanese Examined Patent Publication (Kokoku) No. 5-88690 discloses a method which conducts the reaction in the presence of oxygen by using the catalyst which is obtained by fluorinating a non-Cr-based CoCl.sub.2 /Al.sub.2 O.sub.3, but this catalyst has low activity and low productivity. For these reasons, examinations have so far been made so as to prolong service life of the catalysts. In other words, Japanese Unexamined Patent Publication (Kokai) No. 2-172933 clarifies that a catalyst comprising a halide or oxide containing at least one kind of elements selected from the group consisting of Al, Mg, Ca, Ba, Sr, Fe, Ni, Co and Mn, and containing also Cr, has high durability (long life). EP 502605 discloses a Cr-based catalyst supporting Zn. Further, Japanese Unexamined Patent Publication (Kokai) No. 4-34694 discloses a catalyst comprising Cr.sub.2 O.sub.3 which is partially fluorinated and supports Ru and Pt, and Japanese Unexamined Patent Publication (Kokai) No. 5-269382 discloses a catalyst consisting of chromium oxide and nickel oxide as principal components thereof, as a catalyst having long service life.
However, coking of the catalyst is vigorous in the fluorination reaction of the hydrogen-containing halogenated hydrocarbon as the production method of the HFCs, and even the catalysts described above do not have sufficient life. In other words, it has been necessary in the past to select the reaction condition under which coking difficultly occurs. Because the progress of coking is greater when the ratio of the HF feed quantity to the feed quantity of organic materials (hereinafter called the "molar ratio") is smaller, the progress of coking is retarded by increasing this molar ratio when the conventional fluorination catalyst is used. However, the increase of the molar ratio means the decrease of the feed quantity of the organic materials (when SV is kept constant), and means the drop of STY (space time yield). It can be therefore concluded that catalyst life is prolonged according to the prior art at the sacrifice of STY to a certain extent.
Accordingly, if a catalyst which makes coking more difficult than the conventional catalysts and which has longer service life could be obtained, it becomes possible not only to prolong catalyst life but also to carry out the reaction at a lower molar ratio, so that the improvement of productivity can be also expected.
In the light of the background described above, the present invention aims at providing a fluorination catalyst having long service life in the production of the HFCs, and a method of efficiently producing the HFCs by bringing a halogenated hydrocarbon having 1 to 4 carbon atoms into contact with HF in a gaseous phase by using the catalyst described above.